Well, I suppose nothing is 100%, but it's a bit more than a guess...
Since the harness comes to the rear from the drivers side, the only connections on the passenger side are those to the side marker and tail light assembly. He specifically said "where the reverse light hooks up". That narrows it down to this (see pic).
The gray wire on the tab is in fact the ground for that bulb. Ref. 1974 FSM
Notes-Pictured is backup lamp assembly on 1975 Duster (same as 1974). There is one other tab on the tail light for the ground wire connection to the stop/turn lamp.